Atomistic analysis of stress-dependent activation energy and dominant factors for dislocation nucleation from planar faults in BCC metals: A comparative study to FCC metals

Dislocation nucleation from planar faults, such as grain boundaries and surfaces, is a fundamental plastic deformation event that influences the strength and fracture characteristics of polycrystalline metals and alloys. In this study, the free-end nudged elastic band method is used to determine the stress-dependent activation energy and dominant factors of dislocation nucleation, with edge and screw components, from surfaces (considered as planar faults) in body-centered cubic (BCC) metals. The energy barriers for dislocation nucleation in BCC metals mainly depend on Burgers vector and shear modulus, and they exceed those in FCC metals owing to the different nucleation morphology and affecting factors between BCC and FCC. For BCC Fe, an atomistically-informed model provides temperature and strain-rate-dependent stress for dislocation nucleation from a planar fault, enabling experimental comparison. The predictive model also shows the temperature- and shear stress-dependent frequency map of both dislocation nucleation from planar faults and kink-nucleation of pre-existing screw dislocations. The map indicates the necessity of a large shear stress for the dislocation nucleation and implies the dominant event depending on the dislocation density and applied shear stress. Free energy calculations also reveal temperature-dependent surface formation energy, which is a competitive event to dislocation nucleation at a crack tip under severe stress conditions, often discussed in ductile–brittle transition. The results provide the important fundamentals for dislocation-based modeling and design of BCC metals.